The present invention relates to an electronic high-speed frame pick-up camera with an exposure time and time interval between exposures which are short enough to successively or instantaneously pick up an object whose structure or brightness changes at high speed.
The motion of two-dimensional objects with the elapsing of time can be observed by continuously operating a shutter at high speed so as to obtain a plurality of sequential frames of an image or an instantaneous frame of an image.
There are two types of high speed frame pick-up cameras; one of which obtains a plurality of contiguous image frames by mechanically rotating an optical device. i.e., a mirror or prism at high speed, and the other which them when an electric pulse voltage is applied to the imaging tube so as to generate an electronic image.
The latter or electronic camera, when compared with the former or mechanical camera, provides both exposure time and time intervals between exposures which are shorter than those of the former, and the latter is suitable for taking an image of an object moving at high speed.
FIG. 1 shows a cutaway view of an example of the conventional electronic high-speed frame pick-up camera constructed with an imaging tube.
The construction and operation of the normal type of imaging tube used with this camera will be described hereinafter with reference to FIG. 1.
Optical image 1 of an object being observed is focused on photoelectric layer 4 of an imaging tube through optical lens 2. Responding to the structure and brightness of the optical image being focused on photoelectric layer 4 to be observed, photoelectrons are emitted from the photoelectric layer 4. The optical image of object 1 being observed is converted into photoelectronic image 5 on the surface of the photoelectric layer 4 within a vacuum envelope.
Each portion of photoelectronic image 5 on photoelectric layer 4 emits a number of electrons which is directly proportional to the brightness thereof.
Photoelectronic image 5 is defined as a pattern formed by the distribution of electrons over the entire two-dimensional area on the photoelectric layer 4.
A negative high voltage VK applied to photoelectric layer 4 is negative with respect to another negative high voltage VM applied to mesh electrode 6.
Photoelectrons forming photoelectronic image 5 are accelerated by the potential difference between photoelectric layer 4 and mesh electrode 6 toward mesh electrode 6. The photoelectrons pass through mesh electrode 6 when arriving at the mesh electrode 6.
A negative high voltage applied to focusing electrode 107 is positive with respect to the negative high voltage VK applied to photoelectric layer 4. Anode 108 is kept at the common potential. Electrons move into deflection electrodes 109 after passing through mesh electrode 6.
An optical image on photoelectric layer 4 is converted into the corresponding photoelectronic image in less than one pico second at high speed. Photoelectronic images are generated one after another in accordance with the structure and brightness of the optical images changing with the elapsing of time, and the corresponding photoelectrons are moved one after another toward mesh electrode 6. This results in generation of the photoelectron beams from the photoelectric layer, and in moving of the photoelectron beams along the tube axis toward anode 108.
Two-dimensional information related to the structure and brightness of the optical image at each instant, which is represented by a two-dimensional photoelectron beam density pattern, appears in a plane perpendicular to the tube axis.
A series of two-dimensional photoelectron beam patterns can be seen on the planes perpendicular to the tube axis in a space between deflection electrodes 109a and 109b and the photoelectric layer 4 in such a manner that a pattern generated in a preceding time is arranged in the direction of the deflection electrodes with respect to another pattern generated at a later time.
If a series of the photoelectron patterns are arranged on phosphor layer 111 at time intervals appropriate to picking them up with a frame pick-up camera, a series of images can be obtained from the frame pick-up camera.
Photoelectrons emitted from photoelectric layer 4 with energies of up to the order of electron volts are dispatched at a variety of angles with respect to the photoelectric layer 4. This energy is low compared with that which is obtained by accelerating electrons until they arrive at anode 108 or the energy becomes 10 keV. Electrons constituting an arbitrary point on photoelectric layer 4, i.e., point A on the photoelectronic image, may be defocused while being accelerated toward deflection electrodes 109a and 109b.
An appropriate voltage higher than that applied to the photoelectric layer 4 is applied to focusing electrode 107 so as to constitute an electron lens in a space between the photoelectric layer 4 and the deflection electrodes 109a and 109b. If the photoelectron energies are distributed and if the photoelectrons move in a variety of directions, the spread electrons are concentrated into point A' on phosphor layer 111. These distributions, however, do not cause any problems. Focusing of a photoelectronic image is illustrated by the locuses of electrons in FIG. 1.
Locuses P'.sub.1 and P'.sub.2 in FIG. 1, which correspond to the zero initial velocity photoelectrons generated at points A and B on the photoelectric layer 4, are called the main locuses.
Locuses P'.sub.1 and P'.sub.2 in FIG. 1, which correspond to the .epsilon.o eV initial velocity photoelectrons dispatched from points A and B on the photoelectric layer 4 have angles ranging from -.alpha. to +.alpha.(0.ltoreq..alpha..ltoreq.90.degree.) with respect to the normal lines passing through points A and B, respectively. On locuses P'.sub.1 and P'.sub.2 of the electrons, energies .epsilon.o are in the order of electron volts.
If proper voltages are applied to focusing electrode 107, locuses P'.sub.1 and P'.sub.2 can be intersected by main locuses P.sub.1 and P.sub.2 at points A' and B' on the phosphor layer 111, respectively.
This relation holds for any other points on photoelectronic image 5.
A pair of deflection electrodes 109 are set at the common potential to operate the imaging tube in the normal mode, and they have no effect on the locus of the electrons. Photoelectrons focused on the phosphor layer 111 strike the phosphor layer 111 at high speed and cause scintillation to form an output video signal corresponding to the optical image being input.
If any optical image being input moves beyond the limit to the response of the phosphor material, images formed on phosphor layer 111 are superimposed and no independent image can be displayed. Motion of the optical image being input is thus limited by the responses of the phosphor materials and by the characteristics of the human eye.
The operation of the frame pick-up camera will be described hereinafter.
For operating the imaging tube in the normal mode, DC voltage VK is applied to photoelectric layer 4 and zero voltage is applied to deflection electrodes 109a and 109b so that the deflection electrodes have no effect on the locuses of the electrons. For operating the imaging tube in the frame pick-up mode, voltages changing with time are applied to the photoelectric layer 4 and either deflection electrode 109a or 109b.
FIGS. 2(A) and 2(B) show the operation voltage applied to the photoelectric layer 4 and the deflection voltage applied to the deflection electrode when the imaging tube is operating in the frame pick-up mode.
Square wave voltage W.sub.1 is applied to the photoelectric layer 4 and staircase waveform voltage W.sub.3 is applied to the pair of deflection electrodes 109a and 109b. Let's observe the voltages applied to photoelectric layer 4 and mesh electrode 6. The same negative DC voltage VM as that for operation in the normal mode is applied to mesh electrode 6.
FIG. 2(A) shows the waveform when W.sub.2 =VM. Square wave voltage W.sub.1 with interval T.sub.2, whose potentials are successively specified as voltages V'K(V'K&gt;VM) and VK(VK&lt;VM), is applied to photoelectric layer 4.
An electronic shutter is formed by the potential difference between voltage W.sub.2 (=VM) applied to mesh electrode 6 and voltage W.sub.1 (V'K&gt;VM&gt;VK) applied to photoelectric layer 4.
If voltage W.sub.1 applied to photoelectric layer 4 is V'K, it is higher than voltage VM applied to mesh electrode 6. Electrons emitted from photoelectric layer 4 are reflected from the mesh electrode 6 and thus no image can be obtained from the phosphor layer 111.
If voltage W.sub.1 applied to photoelectric layer 4 is VK, it is lower than voltage VM applied to mesh electrode 6. Electrons emitted from photoelectric layer 4 are accelerated by the mesh electrode 6 and thus enter the space defined by focusing electrode 107.
Period T.sub.1 is defined as the exposure time. The voltage applied to photoelectric layer 4 is specified as VK for exposure T.sub.1, and the electronic shutter opens for this period of time. Period T.sub.2 of square wave voltage W.sub.1 is defined as the time interval between exposures.
Deflection electrode 109b is kept at the common potential which is the same potential as that of the imaging tube operating in the normal mode while such a staircase voltage waveform W.sub.3 as shown in FIG. 2(B) is applied to deflection electrode 109a.
When passing through deflection electrodes 109, the photoelectron beam is deflected in proportion to the deflection voltage applied to the deflection electrodes 109, and it then arrives at phosphor layer 111.
FIG. 3 shows the positional relation of the output images for pictures picked up by the frame pick-up imaging system. When deflection voltage VD.sub.1 is applied to deflection electrodes 109 as shown in FIGS. 2(B), electrons are focused on the location indicated by output image (1) on phosphor layer 111 in FIG. 3, corresponding to the optical image arriving at deflection electrodes 109 at times t.sub.1 through t'.sub.1. When deflection voltage VD.sub.2 (=0) is applied to deflection electrodes 109, electrons are focused on the location indicated by output image (2) on phosphor layer 111 in FIG. 3, corresponding to the optical image arriving at deflection electrodes 109 at time t.sub.2 through t'.sub.2. When deflection voltage VD.sub.3 is applied to deflection electrodes 109, electrons are focused on the location indicated by output image (3) on phosphor layer 111 in FIG. 3, corresponding to the optical image arriving at deflection electrodes 109 at times t.sub.3 through t'.sub.3. These output images (1) through (3) can be picked up by a conventional optical camera 113 as shown in FIG. 1 while the camera shutter is kept open for the time required to arrange the output images. The exposure time T.sub.1 is important to permit picking up the frames of an image by the usual optical camera.
If T.sub.1 is much greater than the time for changing optical image frames, the optical image frames are successively formed at the same locations of the phosphor layer 111 for the exposure time T.sub.1 corresponding to the optical image frames being input. Different image frames are formed on the phosphor layer 111 and this results in inferior space frequency response.
If T.sub.1 is much less than the time for changing optical image frames, an optical image frame forward by the photoelectron beam is cut off after a while and each output image becomes dark. If the optical image frame being input changes at moderate speed, T.sub.1 should be large enough to provide a sufficient intensity within a limit to provide a satisfactory space frequency response. The optical image frames being input are still for time T.sub.1 if T.sub.1 is selected in such a manner as described above. The deflection electrodes are used to arrange the optical image frames on the phosphor layer 111 corresponding to each exposure time. The deflection voltage to cause the photoelectron beam to strike the same location on the phosphor layer should be unchanged at least during exposure time T.sub.1.
Photographic camera 113 is used to record scintillation on the phosphor layer 111. This example shows that three images frames are recorded. A high-speed frame pick-up camera that cannot be realized by optical device can thus be realized by this method. This device, however, is limited in the following point:
The deflection voltage VD in the conventional high-speed frame pick-up camera, which is applied to deflection electrode 109a, must be unchanged during exposure time T.sub.1. This can be realized if the time interval between exposures T.sub.2 is large enough to pick up each image frame. However, the time interval between exposures T.sub.2 cannot easily be decreased to the order of 10 ns or less.
Problems encountered in shortening exposure time T.sub.1 and the time interval between exposures T.sub.2 will be described hereafter referring to FIGS. 4(A) and 4(B). Waveform W.sub.4 in FIG. 4(A) is defined as the voltage waveform which corresponds to such voltage waveform W.sub.1 applied to photoelectric layer 4 as shown in FIG. 2(A), and waveform W.sub.5 in FIG. 4(B) is defined as the voltage waveform which corresponds to such voltage waveform W.sub.3 applied to deflection electrode 109a as shown in FIG. 2(B).
Voltage waveform W.sub.5 should be a staircase-like waveform; however, it is deformed as shown in FIG. 4(B) and this deformation makes the spatial frequency response inferior.
When exposure time T.sub.1 is of the order of 10 ns, waveform W.sub.4 to be applied to the photoelectric layer 4 is deformed as shown in FIG. 4(A). When the potential on the photoelectric layer 4 is negative compared with that of the mesh electrode 6, the photoelectron beam causes the phosphor layer 111 to scintillate passing through the mesh electrode 6.
When the potential on the photoelectric layer 4 is other than VK at any point of time on such a potential gradient as shown in FIG. 4(A), scintillation can occur in the phosphor layer 111. An electron lens formed by the potential gradient has a capability to form a photoelectronic image on the phosphor layer 111 by focusing of the electron beam only when the potential on the photoelectric layer 4 is kept at VK. The photoelectronic image may be defocused at any other potential than VK. This means that such a waveform as shown in FIG. 4(A) may cause the spatial frequency response of the output image to be inferior.
Voltage waveform W.sub.4, even if deformed, has an amplitude of the order of 10 to 100 volts. Voltage waveform W.sub.5, even if deformed, has an amplitude of the order of 10 to 100 volts with a DC component of 1 to 2 kV. If T.sub.1 and/or T.sub.2 are/is of the order of 10 ns or less, an ideal circuit to generate such an ideal signal waveform as shown in FIGS. 2(A) and 2(B) cannot be constructed.
FIG. 5 shows a cutaway view of another example of the conventional electronic high-speed frame pick-up camra constructed with an imaging tube. Construction and operation of the normal type of imaging tube of this conventional camera will be described hereafter.
Optical image 1 of an object being observed is focused on photoelectric layer 4 of an imaging tube through optical lens 2. Responding to the structure and brightness of the optical image being focused on photoelectric layer 4 to be observed, photoelectrons are emitted from the photoelectric layer 4. The optical image of object 1 being observed is converted into photoelectronic image 5 on the surface of the photoelectric layer 4 within a vacuum envelope.
Each portion of photoelectronic image 5 on photoelectric layer 4 emits a number of electrons which is directly proportional to the brightness thereof. Photoelectronic image 5 is defined as a pattern formed by the distribution of electrons over the entire two-dimensional area on the photoelectric layer 4.
A negative high voltage VK applied to photoelectric layer 4 is negative with respect to another negative high voltage VM applied to mesh electrode 6.
Photoelectrons forming photoelectronic image 5 are accelerated by the potential difference between photoelectric layer 4 and mesh electrode 6 toward mesh electrode 6. The photoelectrons pass through mesh electrode 6 when arriving at the mesh electrode 6.
A negative high voltage applied to focusing electrode 107 is positive with respect to another negative high voltage VK applied to photoelectric layer 4. Anode 108 and phosphor layer 111 are kept at the common potential. Electrons move into phosphor layer 111 after passing through mesh electrode 6.
An optical image on photoelectric layer 4 is converted into the corresponding photoelectronic image in less than one pico second at high speed. Photoelectronic images are generated one after another in accordance with the structure and brightness of the optical images changing with the elapsing of time, and the corresponding photoelectrons are moved one after another toward mesh electrode 6. This results in generation of the photoelectron beams from the photoelectric layer, and in moving of the photoelectron beams along the tube axis toward phosphor layer 111.
Two-dimensional information related to the structure and brightness of the optical image at each time, which is represented by a two-dimensional photoelectron beam density pattern, appears in a plane perpendicular to the tube axis. A series of two-dimensional photoelectron beam patterns can be seen on the planes perpendicular to the tube axis in the space between phosphor lay 111 and the photoelectric layer 4 in such a manner that a pattern generated in a preceding time is arranged in the direction of the phosphor layer 111 with respect to another pattern generated at a later time. If the photoelectron pattern is arranged on phosphor layer 111 at a time appropriate for picking it up with a frame pick-up camera, an image frame can be obtained from the frame pick-up camera.
Photoelectrons emitted from photoelectric layer 4 with energies of up to the order of electron volts are dispatched at a variety of angles with respect to the photoelectric layer 4. This energy is low compared with that which is obtained by accelerating electrons until they arrive at anode 108 or the energy becomes 10 keV. Electrons constituting an arbitrary point on photoelectric layer 4, i.e., point A on the photoelectronic image, may be defocused while being accelerated toward anode 108.
An appropriate voltage higher than that applied to the photoelectric layer 4 is applied to focusing electrode 107 so as to constitute an electron lens in the space between the photoelectric layer 4 and the phosphor layer 111. If the photoelectron energies are distributed and if the photoelectrons move in a variety of directions, the spread electrons are concentrated into points A' on phosphor layer 111. These distributions, however, do not cause any problems. Focusing of a photoelectronic image is illustrated by the locuses of the electrons in FIG. 5.
Locuses P.sub.1 and P.sub.2 in FIG. 5, which correspond to the zero initial velocity photoelectrons generated at points A and B on the photoelectric layer 4, are called the main locuses.
Locuses P'.sub.1 and P'.sub.2 in FIG. 5 which correspond to the .epsilon.o eV initial velocity photoelectrons dispatched from points A and B on the photoelectric layer 4 at angles ranging from -.alpha. to +.alpha.(0.ltoreq..alpha..ltoreq.90.degree.) with respect to the normal lines passing through points A and B, respectively. On locuses P'.sub.1 and P'.sub.2 of electrons, energies .epsilon.o are in the order of electron volts.
If proper voltages are applied to focusing electrode 107, locuses P'.sub.1 and P'.sub.2 can be intersected by main locuses P.sub.1 and P.sub.2 at points A' and B' on the phosphor layer 111, respectively. This relation holds for any other points on photoelectronic image 5. Photoelectrons focused on the phosphor layer 111 strike the phosphor layer 111 at high speed and cause scintillation to form an output video signal corresponding to the optical image being input. If any optical image being input moves beyond the limit of the response of the phosphor material, images formed on phosphor, layer 111 are superimposed and no independent image can be display.
The motion of the optical image being input is thus limited by the responses of the phosphor materials and the characteristics of the human eye.
The operation of this frame pick-up camera will be described hereinafter.
For operating the imaging tube in the normal mode, DC voltage VK is applied to photoelectric layer 4. For operating the imaging tube in the electronic shutter mode, the voltage applied to the photoelectric layer 4 is changed.
FIG. 6 shows the operation voltage applied to the photoelectric layer 4 when the imaging tube is operating in the electronic shutter mode. Square wave voltage W.sub.1 is applied to the photoelectric layer 4. Let's observe the voltages applied to the photoelectric layer 4 and mesh electrode 6.
The same negative DC voltage VM as that for operations in the normal mode is applied to mesh electrode 6.
FIG. 6 show the waveforms when W.sub.2 =VM. Square wave voltage W.sub.1 whose potentials are successively specified as voltages V'K(V'K&gt;VM) and VK(VK&lt;VM) is applied to photoelectric layer 4.
An electronic shutter is formed by the potential difference between the voltage W.sub.2 (=VM) applied to mesh electrode 6 and voltage W.sub.1 (V'K&gt;VM&gt;VK) applied to photoelectric layer 4. IF voltage W.sup.1 applied to photoelectric layer 4 is V'K, it is higher than voltage VM applied to mesh electrode 6. Electrons emitted from photoelectric layer 4 are reflected from the mesh electrode 6 and thus no image can be obtained from the phosphor layer 111.
If voltage W.sub.1 applied to photoelectric layer 4 is VK, it is lower than voltage VM applied to mesh electrode 6. Electrons emitted from photoelectric layer 4 are accelerated by the mesh electrode 6 and thus enter the space defined by focusing electrode 107.
Period T.sub.1 is defined as the exposure time. The voltage applied to photoelectric layer 4 is specified as VK for exposure time T.sub.1, and the electronic shutter opens for this period of time.
Electrons emitted during the exposure time can only be focused on the phosphor layer 4 and scintillation occurs in the phosphor layer 4. The time of scintillation, depending on the types of phosphors, is a short period in the order of 10 .mu.s to 1 ms.
The output image can be picked up by such optical camera 113 as shown in FIG. 5 while the camera shutter is kept open.
The exposure time T.sub.1 is important to pick up an image frame by the usual optical camera.
If T.sub.1 is much greater than the time for changing optical image frames, the optical image frames are successively formed on the same locations of the phosphor layer 111 for exposure time T.sub.1 corresponding to the optical image frames being input. Different image frames are formed on the phosphor layer 111 and this results in inferior space frequency response.
If T.sub.1 is much less than the time for changing optical image frames, an optical image frame formed by the photoelectron beam is cut off after a while and each output image becomes dark. If the optical image frame being input changes at moderate speed, T.sub.1 should be large enough to provide a sufficient intensity within a limit providing a satisfactory space frequency response. The optical image frames being input are still for time T.sub.1 if T.sub.1 is selected in such a manner as described above.
Such a high-speed frame pick-up camera that cannot be realized by an optical device can thus be realized by this method. This device, however, is limited in the following point:
Problems encountered in a short exposure time (T.sub.1) will be explained hereafter referring to FIG. 7. In FIG. 7, voltage waveform W.sub.1 corresponds to voltage waveform W.sub.1 applied to photoelectric layer 4 in FIG. 6.
When the exposure time T.sub.1 is of the order of 10 ns or less, waveform W.sub.1 to be applied to the photoelectric layer 4 is deformed as shown in FIG. 7.
When the potential on the photoelectric layer 4 is negative compared with that of the mesh electrode 6, the photoelectron beam causes the phosphor layer 111 to scintillate passing through the mesh electrode 6.
When the potential on the photoelectric layer 4 is other than VK at any point of time on such a potential gradient as shown in FIG. 7, scintillation can occur in the phosphor layer 111.
An electron lens formed by the potential gradient has a capability to form a photoelectronic image on the phosphor layer 111 by focusing of the electron beam only when the potential on the photoelectric layer 4 is kept at VK. The photoelectronic image may be defocused at any other potential than VK. This means that such a waveform as shown in FIG. 7 may cause the spatial frequency response of the output image to be inferior.
Voltage waveform W.sub.1 of FIG. 7, even if deformed, has an amplitude of the order of 10 to 100 volts. If T.sub.1 is of the order of 10 ns or less, an ideal circuit to generate such an ideal signal waveform as shown in FIG. 6 cannot be constructed.
An objective of the present invention is to present a new type of high-speed, frame pick-up camera capable of picking up a plurality of successive image frames which may change at high speed.
Another objective of the present invention is to present a new type of high-speed, frame pick-up camera capable of picking up an instantaneous image frame which may change at high speed.